5.5.2 Particular Function Checks

These macros check for particular C functions—whether they exist, and
in some cases how they respond when given certain arguments.

Macro: AC_FUNC_ALLOCA

Check how to get alloca. Tries to get a builtin version by
checking for ‘alloca.h’ or the predefined C preprocessor macros
__GNUC__ and _AIX. If this macro finds ‘alloca.h’,
it defines HAVE_ALLOCA_H.

If those attempts fail, it looks for the function in the standard C
library. If any of those methods succeed, it defines
HAVE_ALLOCA. Otherwise, it sets the output variable
ALLOCA to ‘${LIBOBJDIR}alloca.o’ and defines
C_ALLOCA (so programs can periodically call ‘alloca (0)’ to
garbage collect). This variable is separate from LIBOBJS so
multiple programs can share the value of ALLOCA without needing
to create an actual library, in case only some of them use the code in
LIBOBJS. The ‘${LIBOBJDIR}’ prefix serves the same
purpose as in LIBOBJS (see section AC_LIBOBJ vs. LIBOBJS).

This macro does not try to get alloca from the System V R3
‘libPW’ or the System V R4 ‘libucb’ because those libraries
contain some incompatible functions that cause trouble. Some versions
do not even contain alloca or contain a buggy version. If you
still want to use their alloca, use ar to extract
‘alloca.o’ from them instead of compiling ‘alloca.c’.

Source files that use alloca should start with a piece of code
like the following, to declare it properly.

If the chown function is available and works (in particular, it
should accept ‘-1’ for uid and gid), define
HAVE_CHOWN. The result of this macro is cached in the
ac_cv_func_chown_works variable.

Macro: AC_FUNC_CLOSEDIR_VOID

If the closedir function does not return a meaningful value,
define CLOSEDIR_VOID. Otherwise, callers ought to check its
return value for an error indicator.

Currently this test is implemented by running a test program. When
cross compiling the pessimistic assumption that closedir does not
return a meaningful value is made.

The result of this macro is cached in the ac_cv_func_closedir_void
variable.

This macro is obsolescent, as closedir returns a meaningful value
on current systems. New programs need not use this macro.

Macro: AC_FUNC_ERROR_AT_LINE

If the error_at_line function is not found, require an
AC_LIBOBJ replacement of ‘error’.

The result of this macro is cached in the ac_cv_lib_error_at_line
variable.

The AC_FUNC_ERROR_AT_LINE macro is obsolescent. New programs
should use Gnulib’s error module. See section Gnulib.

Macro: AC_FUNC_FNMATCH

If the fnmatch function conforms to Posix, define
HAVE_FNMATCH. Detect common implementation bugs, for example,
the bugs in Solaris 2.4.

Unlike the other specific
AC_FUNC macros, AC_FUNC_FNMATCH does not replace a
broken/missing fnmatch. This is for historical reasons.
See AC_REPLACE_FNMATCH below.

The result of this macro is cached in the ac_cv_func_fnmatch_works
variable.

This macro is obsolescent. New programs should use Gnulib’s
fnmatch-posix module. See section Gnulib.

Macro: AC_FUNC_FNMATCH_GNU

Behave like AC_REPLACE_FNMATCH (replace) but also test
whether fnmatch supports GNU extensions. Detect common
implementation bugs, for example, the bugs in the GNU C
Library 2.1.

The result of this macro is cached in the ac_cv_func_fnmatch_gnu
variable.

This macro is obsolescent. New programs should use Gnulib’s
fnmatch-gnu module. See section Gnulib.

Macro: AC_FUNC_FORK

This macro checks for the fork and vfork functions. If a
working fork is found, define HAVE_WORKING_FORK. This macro
checks whether fork is just a stub by trying to run it.

If ‘vfork.h’ is found, define HAVE_VFORK_H. If a working
vfork is found, define HAVE_WORKING_VFORK. Otherwise,
define vfork to be fork for backward compatibility with
previous versions of autoconf. This macro checks for several known
errors in implementations of vfork and considers the system to not
have a working vfork if it detects any of them. It is not considered
to be an implementation error if a child’s invocation of signal
modifies the parent’s signal handler, since child processes rarely change
their signal handlers.

Since this macro defines vfork only for backward compatibility with
previous versions of autoconf you’re encouraged to define it
yourself in new code:

#ifndef HAVE_WORKING_VFORK
# define vfork fork
#endif

The results of this macro are cached in the ac_cv_func_fork_works
and ac_cv_func_vfork_works variables. In order to override the
test, you also need to set the ac_cv_func_fork and
ac_cv_func_vfork variables.

Macro: AC_FUNC_FSEEKO

If the fseeko function is available, define HAVE_FSEEKO.
Define _LARGEFILE_SOURCE if necessary to make the prototype
visible on some systems (e.g., glibc 2.2). Otherwise linkage problems
may occur when compiling with AC_SYS_LARGEFILE on
largefile-sensitive systems where off_t does not default to a
64bit entity. All systems with fseeko also supply ftello.

Macro: AC_FUNC_GETGROUPS

If the getgroups function is available and works (unlike on
Ultrix 4.3, where ‘getgroups (0, 0)’ always fails), define
HAVE_GETGROUPS. Set GETGROUPS_LIBS to any libraries
needed to get that function. This macro runs AC_TYPE_GETGROUPS.

Macro: AC_FUNC_GETLOADAVG

Check how to get the system load averages. To perform its tests
properly, this macro needs the file ‘getloadavg.c’; therefore, be
sure to set the AC_LIBOBJ replacement directory properly (see
Generic Function Checks, AC_CONFIG_LIBOBJ_DIR).

If the system has the getloadavg function, define
HAVE_GETLOADAVG, and set GETLOADAVG_LIBS to any libraries
necessary to get that function. Also add GETLOADAVG_LIBS to
LIBS. Otherwise, require an AC_LIBOBJ replacement for
‘getloadavg’ with source code in ‘dir/getloadavg.c’, and
possibly define several other C preprocessor macros and output
variables:

Define C_GETLOADAVG.

Define SVR4, DGUX, UMAX, or UMAX4_3 if on
those systems.

If ‘nlist.h’ is found, define HAVE_NLIST_H.

If ‘struct nlist’ has an ‘n_un.n_name’ member, define
HAVE_STRUCT_NLIST_N_UN_N_NAME. The obsolete symbol
NLIST_NAME_UNION is still defined, but do not depend upon it.

Programs may need to be installed set-group-ID (or set-user-ID) for
getloadavg to work. In this case, define
GETLOADAVG_PRIVILEGED, set the output variable NEED_SETGID
to ‘true’ (and otherwise to ‘false’), and set
KMEM_GROUP to the name of the group that should own the installed
program.

The AC_FUNC_GETLOADAVG macro is obsolescent. New programs should
use Gnulib’s getloadavg module. See section Gnulib.

Macro: AC_FUNC_GETMNTENT

Check for getmntent in the standard C library, and then in the
‘sun’, ‘seq’, and ‘gen’ libraries, for UNICOS,
IRIX 4, PTX, and UnixWare, respectively. Then, if
getmntent is available, define HAVE_GETMNTENT and set
ac_cv_func_getmntent to yes. Otherwise set
ac_cv_func_getmntent to no.

The result of this macro can be overridden by setting the cache variable
ac_cv_search_getmntent.

Macro: AC_FUNC_GETPGRP

Define GETPGRP_VOID if it is an error to pass 0 to
getpgrp; this is the Posix behavior. On older BSD
systems, you must pass 0 to getpgrp, as it takes an argument and
behaves like Posix’s getpgid.

#ifdef GETPGRP_VOID
pid = getpgrp ();
#else
pid = getpgrp (0);
#endif

This macro does not check whether
getpgrp exists at all; if you need to work in that situation,
first call AC_CHECK_FUNC for getpgrp.

The result of this macro is cached in the ac_cv_func_getpgrp_void
variable.

This macro is obsolescent, as current systems have a getpgrp
whose signature conforms to Posix. New programs need not use this macro.

Macro: AC_FUNC_LSTAT_FOLLOWS_SLASHED_SYMLINK

If ‘link’ is a symbolic link, then lstat should treat
‘link/’ the same as ‘link/.’. However, many older
lstat implementations incorrectly ignore trailing slashes.

It is safe to assume that if lstat incorrectly ignores
trailing slashes, then other symbolic-link-aware functions like
unlink also incorrectly ignore trailing slashes.

The result of this macro is cached in the
ac_cv_func_lstat_dereferences_slashed_symlink variable.

The AC_FUNC_LSTAT_FOLLOWS_SLASHED_SYMLINK macro is obsolescent.
New programs should use Gnulib’s lstat module. See section Gnulib.

Macro: AC_FUNC_MALLOC

If the malloc function is compatible with the GNU C
library malloc (i.e., ‘malloc (0)’ returns a valid
pointer), define HAVE_MALLOC to 1. Otherwise define
HAVE_MALLOC to 0, ask for an AC_LIBOBJ replacement for
‘malloc’, and define malloc to rpl_malloc so that the
native malloc is not used in the main project.

Typically, the replacement file ‘malloc.c’ should look like (note
the ‘#undef malloc’):

The result of this macro is cached in the
ac_cv_func_malloc_0_nonnull variable.

Macro: AC_FUNC_MBRTOWC

Define HAVE_MBRTOWC to 1 if the function mbrtowc and the
type mbstate_t are properly declared.

The result of this macro is cached in the ac_cv_func_mbrtowc
variable.

Macro: AC_FUNC_MEMCMP

If the memcmp function is not available, or does not work on
8-bit data (like the one on SunOS 4.1.3), or fails when comparing 16
bytes or more and with at least one buffer not starting on a 4-byte
boundary (such as the one on NeXT x86 OpenStep), require an
AC_LIBOBJ replacement for ‘memcmp’.

The result of this macro is cached in the
ac_cv_func_memcmp_working variable.

This macro is obsolescent, as current systems have a working
memcmp. New programs need not use this macro.

Macro: AC_FUNC_MKTIME

If the mktime function is not available, or does not work
correctly, require an AC_LIBOBJ replacement for ‘mktime’.
For the purposes of this test, mktime should conform to the
Posix standard and should be the inverse of
localtime.

The result of this macro is cached in the
ac_cv_func_working_mktime variable.

The AC_FUNC_MKTIME macro is obsolescent. New programs should
use Gnulib’s mktime module. See section Gnulib.

Macro: AC_FUNC_MMAP

If the mmap function exists and works correctly, define
HAVE_MMAP. This checks only private fixed mapping of already-mapped
memory.

The result of this macro is cached in the
ac_cv_func_mmap_fixed_mapped variable.

Macro: AC_FUNC_OBSTACK

If the obstacks are found, define HAVE_OBSTACK, else require an
AC_LIBOBJ replacement for ‘obstack’.

The result of this macro is cached in the ac_cv_func_obstack
variable.

Macro: AC_FUNC_REALLOC

If the realloc function is compatible with the GNU C
library realloc (i.e., ‘realloc (NULL, 0)’ returns a
valid pointer), define HAVE_REALLOC to 1. Otherwise define
HAVE_REALLOC to 0, ask for an AC_LIBOBJ replacement for
‘realloc’, and define realloc to rpl_realloc so that
the native realloc is not used in the main project. See
AC_FUNC_MALLOC for details.

The result of this macro is cached in the
ac_cv_func_realloc_0_nonnull variable.

Macro: AC_FUNC_SELECT_ARGTYPES

Determines the correct type to be passed for each of the
select function’s arguments, and defines those types
in SELECT_TYPE_ARG1, SELECT_TYPE_ARG234, and
SELECT_TYPE_ARG5 respectively. SELECT_TYPE_ARG1 defaults
to ‘int’, SELECT_TYPE_ARG234 defaults to ‘int *’,
and SELECT_TYPE_ARG5 defaults to ‘struct timeval *’.

This macro is obsolescent, as current systems have a select whose
signature conforms to Posix. New programs need not use this macro.

Macro: AC_FUNC_SETPGRP

If setpgrp takes no argument (the Posix version), define
SETPGRP_VOID. Otherwise, it is the BSD version, which takes
two process IDs as arguments. This macro does not check whether
setpgrp exists at all; if you need to work in that situation,
first call AC_CHECK_FUNC for setpgrp.

The result of this macro is cached in the ac_cv_func_setpgrp_void
variable.

This macro is obsolescent, as current systems have a setpgrp
whose signature conforms to Posix. New programs need not use this macro.

Macro: AC_FUNC_STAT

Macro: AC_FUNC_LSTAT

Determine whether stat or lstat have the bug that it
succeeds when given the zero-length file name as argument. The stat
and lstat from SunOS 4.1.4 and the Hurd (as of 1998-11-01) do
this.

If it does, then define HAVE_STAT_EMPTY_STRING_BUG (or
HAVE_LSTAT_EMPTY_STRING_BUG) and ask for an AC_LIBOBJ
replacement of it.

The results of these macros are cached in the
ac_cv_func_stat_empty_string_bug and the
ac_cv_func_lstat_empty_string_bug variables, respectively.

These macros are obsolescent, as no current systems have the bug.
New programs need not use these macros.

Macro: AC_FUNC_STRCOLL

If the strcoll function exists and works correctly, define
HAVE_STRCOLL. This does a bit more than
‘AC_CHECK_FUNCS(strcoll)’, because some systems have incorrect
definitions of strcoll that should not be used.

The result of this macro is cached in the ac_cv_func_strcoll_works
variable.

Macro: AC_FUNC_STRERROR_R

If strerror_r is available, define HAVE_STRERROR_R, and if
it is declared, define HAVE_DECL_STRERROR_R. If it returns a
char * message, define STRERROR_R_CHAR_P; otherwise it
returns an int error number. The Thread-Safe Functions option of
Posix requires strerror_r to return int, but
many systems (including, for example, version 2.2.4 of the GNU C
Library) return a char * value that is not necessarily equal to
the buffer argument.

The result of this macro is cached in the
ac_cv_func_strerror_r_char_p variable.

Macro: AC_FUNC_STRFTIME

Check for strftime in the ‘intl’ library, for SCO Unix.
Then, if strftime is available, define HAVE_STRFTIME.

This macro is obsolescent, as no current systems require the ‘intl’
library for strftime. New programs need not use this macro.

Macro: AC_FUNC_STRTOD

If the strtod function does not exist or doesn’t work correctly,
ask for an AC_LIBOBJ replacement of ‘strtod’. In this case,
because ‘strtod.c’ is likely to need ‘pow’, set the output
variable POW_LIB to the extra library needed.

This macro caches its result in the ac_cv_func_strtod variable
and depends upon the result in the ac_cv_func_pow variable.

The AC_FUNC_STRTOD macro is obsolescent. New programs should
use Gnulib’s strtod module. See section Gnulib.

Macro: AC_FUNC_STRTOLD

If the strtold function exists and conforms to C99, define
HAVE_STRTOLD.

This macro caches its result in the ac_cv_func_strtold variable.

Macro: AC_FUNC_STRNLEN

If the strnlen function is not available, or is buggy (like the one
from AIX 4.3), require an AC_LIBOBJ replacement for it.

This macro caches its result in the ac_cv_func_strnlen_working
variable.

This macro is obsolescent, as all current systems have a utime
that behaves this way. New programs need not use this macro.

Macro: AC_FUNC_VPRINTF

If vprintf is found, define HAVE_VPRINTF. Otherwise, if
_doprnt is found, define HAVE_DOPRNT. (If vprintf
is available, you may assume that vfprintf and vsprintf
are also available.)

This macro is obsolescent, as all current systems have vprintf.
New programs need not use this macro.

Macro: AC_REPLACE_FNMATCH

If the fnmatch function does not conform to Posix (see
AC_FUNC_FNMATCH), ask for its AC_LIBOBJ replacement.

The files ‘fnmatch.c’, ‘fnmatch_loop.c’, and ‘fnmatch_.h’
in the AC_LIBOBJ replacement directory are assumed to contain a
copy of the source code of GNU fnmatch. If necessary,
this source code is compiled as an AC_LIBOBJ replacement, and the
‘fnmatch_.h’ file is linked to ‘fnmatch.h’ so that it can be
included in place of the system <fnmatch.h>.

This macro caches its result in the ac_cv_func_fnmatch_works
variable.

This macro is obsolescent, as it assumes the use of particular source
files. New programs should use Gnulib’s fnmatch-posix module,
which provides this macro along with the source files. See section Gnulib.